DATA SEGMENT 
A DB 5FH
B DB 3FH
C DB '?'
E DD '?'
DATA  ENDS 

CODE SEGMENT
ASSUME CS:CODE,DS:DATA
START:
MOV DX,DATA
MOV DS,DX
MOV AX,0
MOV ES,AX
MOV AX,OFFSET CHANGE
MOV WORD PTR ES:[80H],AX
MOV AX,SEG CHANGE
MOV WORD PTR ES:[82H],AX
INT 20H
JMP s
CHANGE PROC NEAR
XOR AX,AX
XOR BX,BX
MOV AL,A
MOV BL,B
ADD AL,BL
MOV C,AL	
IRET
CHANGE ENDP 
CODE ENDS
S: 
END START